Window: btoa() method - Web APIs | MDN (original) (raw)

Syntax

Parameters

The binary string to encode. Strings in JavaScript are encoded as UTF-16, so this means each character must have a code point less than 256, representing one byte of data.

Return value

An ASCII string containing the Base64 representation of stringToEncode.

Exceptions

InvalidCharacterError DOMException

The string contained a character that did not fit in a single byte. See "Unicode strings" below for more detail.

Examples

const encodedData = window.btoa("Hello, world"); // encode a string
const decodedData = window.atob(encodedData); // decode the string

Base64, by design, expects binary data as its input. In terms of JavaScript strings, this means strings in which the code point of each character occupies only one byte. So if you pass a string into btoa() containing characters that occupy more than one byte, you will get an error, because this is not considered binary data:

const ok = "a";
console.log(ok.codePointAt(0).toString(16)); //   61: occupies < 1 byte

const notOK = "✓";
console.log(notOK.codePointAt(0).toString(16)); // 2713: occupies > 1 byte

console.log(window.btoa(ok)); // YQ==
console.log(window.btoa(notOK)); // error

Since btoa interprets the code points of its input string as byte values, calling btoa on a string will cause a "Character Out Of Range" exception if a character's code point exceeds 0xff. For use cases where you need to encode arbitrary Unicode text, it is necessary to first convert the string to its constituent bytes in UTF-8, and then encode the bytes.

The simplest solution is to use TextEncoder and TextDecoder to convert between UTF-8 and single-byte representations of the string:

function base64ToBytes(base64) {
  const binString = atob(base64);
  return Uint8Array.from(binString, (m) => m.codePointAt(0));
}

function bytesToBase64(bytes) {
  const binString = Array.from(bytes, (byte) =>
    String.fromCodePoint(byte),
  ).join("");
  return btoa(binString);
}

// Usage
bytesToBase64(new TextEncoder().encode("a Ā 𐀀 文 🦄")); // "YSDEgCDwkICAIOaWhyDwn6aE"
new TextDecoder().decode(base64ToBytes("YSDEgCDwkICAIOaWhyDwn6aE")); // "a Ā 𐀀 文 🦄"

Converting arbitrary binary data

The bytesToBase64 and base64ToBytes functions in the previous section can be used directly to convert between Base64 strings and Uint8Arrays.

For better performance, asynchronous conversion between base64 data URLs is possible natively within the web platform via the FileReader and fetch APIs:

async function bytesToBase64DataUrl(bytes, type = "application/octet-stream") {
  return await new Promise((resolve, reject) => {
    const reader = Object.assign(new FileReader(), {
      onload: () => resolve(reader.result),
      onerror: () => reject(reader.error),
    });
    reader.readAsDataURL(new File([bytes], "", { type }));
  });
}

async function dataUrlToBytes(dataUrl) {
  const res = await fetch(dataUrl);
  return new Uint8Array(await res.arrayBuffer());
}

// Usage
await bytesToBase64DataUrl(new Uint8Array([0, 1, 2])); // "data:application/octet-stream;base64,AAEC"
await dataUrlToBytes("data:application/octet-stream;base64,AAEC"); // Uint8Array [0, 1, 2]

Specifications

Specification
HTML # dom-btoa-dev